That's a good point, which I haven't really thought about. Why the hell would anyone go and become insane on purpose? I mean if we can't RP as Void Elves who has controlled the power of the Void, and all Void Elves are batshit crazy, who would actually bolster the ranks of Void Elves? If a Blood Elf/High Elf have mastered the arcane magic and are in control over that, why would they ever risk the life of going insane for no power increase?
I don't think the devs have thought this one through. The population argument against High Elves just got even weaker, if that was possible. Not that I think the devs care one bit about consistency when it comes to the story.